A synchronous generator consists of two windings, which are an armature winding and a magnetic field winding. The armature winding is normally a 3-phase AC static winding, so it is also called a stator winding, and the magnetic field winding is normally a DC rotating winding, which is also known as a rotor winding. The DC current flowing through the rotor winding is also called the rotor current, the rotor current produces a magnetic field that rotates with the rotor in the rotor winding, the rotating magnetic field cuts the stator winding to induce the stator voltage, after being connected to the grid, the generator delivers the active power to the grid through the stator, exchanges reactive power with the grid at the same time, and flows current through the stator winding, which is called stator current. When the synchronous generator is running, the active power is determined by the mechanical power input by the prime mover, and the reactive power is regulated by the rotor current output from the excitation system, wherein the reactive power will cause the stator current to increase, and cause the internal loss of the generator to increase, which will cause the internal temperature of the stator to rise. Since the heat is in an extremely inverse time relationship with the stator current, the larger the stator current is, the faster the stator winding temperature rises, and the shorter the running time of the stator current can be allowed. If the overcurrent level of the generator stator winding cannot be controlled in real time and accurately, when the stator temperature exceeds the allowable upper temperature limit of the stator, the generator stator will be overheated and damaged, resulting in huge direct economic loss and indirect economic loss.
Since the active power has nothing to do with the excitation regulation, in the generator operation, the excitation system mainly controls the generator stator current by adjusting the reactive power of the generator to ensure the safe operation of the generator stator. The generator excitation system is a general term for all equipment that supplies and regulates the magnetic field current. At present, the generator excitation system is generally equipped with the functions of stator overcurrent limiting and reactive power overexcitation limiting, wherein the process of the stator overcurrent limiting is as follows that the excitation regulator compares the stator current measured in real time with the predetermined stator overcurrent limit setting value, when the actual stator current and heat generation are greater than the limit setting value, the stator overcurrent limiting function is activated, if the reactive power of the generator is greater than 0, reduces the magnetic field current, or if the reactive power of the generator is less than 0, increases the magnetic field current of the synchronous generator to achieve the goal of adjusting the generator stator current; the process of the reactive power overexcitation limiting is as follows that the excitation regulator compares the reactive power of the generator measured in real time with the predetermined reactive power overexcitation limit setting value, when the reactive power is greater than the limit setting value, the reactive power overexcitation limiting function is activated after a fixed time delay (usually 20 seconds), adjusts the reactive power back to the limit setting value to ensure that the generator operates within the predetermined reactive power range.
The function of reactive power overexcitation limiting configured in the above excitation system has been too simplified by the fact that the setting of the reactive power overexcitation limit value is relatively simple, which is mainly determined according to the reactive power range of the generator during operation, and that when the reactive power exceeds the limit setting value, the regulation is simply activated after a delay to adjust the reactive power of the generator back to the limit setting value. The excitation regulation of the generator is very important to stabilize the performance of the grid when the generator is connected to the grid, it is therefor necessary to give full play to the stable support capacity of the generator to the power grid on the basis of ensuring the safety of the generator. The reactive power range and time of the generator are related to the heating of the armature winding of the generator, and are also related to the operation of the grid, in addition the reactive power limit violation and the allowable operating time should also be related to the reactive power. At present, the reactive power limiting only serves to inform the operating personnel of the excessive reactive power.
The function of stator overcurrent limiting can work normally under normal operating conditions of the generator and the grid (the voltage is at the rated value, and the active power does not exceed the rated value), the reason for the increase of the stator current is mainly due to the magnetic field current limit violation (too large or too small), at this time, the goal of regulating the stator current to the limit setting value can be achieved by regulating the magnetic field current. However, in some special operating conditions, such as low voltage caused by the grid faults, or large active power caused by the generator governor faults, after the function of the stator overcurrent limiting is activated, the stator current cannot be regulated to a predetermined goal, and the magnetic field current, reactive power, and stator voltage may oscillate greatly during the regulating process. Based on engineering experience, some technicians have noticed that this phenomenon will occur when the reactive power of the synchronous generator is small, in most generator excitation systems, the reactive power is therefore added as a condition to the stator overcurrent limiting function of the synchronous generator, when the reactive power is greater than a certain threshold, the stator overcurrent limiting of the synchronous generator is activated. However, there are operation practice proves that increasing the reactive power condition cannot solve the above phenomenon in essence.
In summary, the inventors are committed to researching and improving the reactive power overexcitation limit regulating function of the generator, solving the startup logic and the regulating goal of the reactive power overexcitation limiting function, and solving the phenomenon and problem that the field current, the reactive power, and the armature voltage oscillate greatly when the stator current is limited to a specific operating condition. The problems that need to be solved can be summarized as the following.
(I) Researching the problem of determining the allowable value of reactive power in actual operation, and researching the essential cause of the phenomenon and problem that the stator overcurrent limiting starts the field current, the reactive power, and the armature voltage to oscillate greatly;
(II) Developing targeted technologies and methods to ensure that the reactive power output of synchronous generator is not only beneficial to reduce generator stator heating to protect generator safety, but also beneficial to support the grid voltage to improve the stability of the power system performance under any operating conditions, and solving, at the same time, the problem that the stator overcurrent limiting starts the field current, the reactive power, and the armature voltage to oscillate greatly under certain operating conditions;
(III) Developing practical technical solutions, and proposing a realization method and an action model of a novel reactive power overexcitation limiting regulation technology for generator.